Ink jet printing compositions and methods

ABSTRACT

An ink composition is provided which is suitable for use in an ink jet printer and comprises solvent, colourant and binder, wherein the solvent comprises methyl acetate. The composition is particularly suitable for printing on polymeric substrates, eg polypropylenes and polyethylenes.

The present application is a continuation of Application 09/961,153,filed Sep. 21, 2001 now U.S. Pat. No. 6,814,791. The disclosure of theprior application by its entirety is incorporated herein by reference.Application 09/961,153 claims the benefit of the foreign filing date forEuropean 00308558.6 filed Sep. 29, 2000.

FIELD OF THE INVENTION

This invention relates to compositions suitable for ink jet printing andto methods in which these compositions are used for printing onsubstrates.

BACKGROUND OF THE INVENTION

Ink jet printing methods are well known for application of compositionsto a wide range of substrates. Ink jet printing methods are versatileand particularly valuable for printing indicia on substrates used inhigh speed processes. Examples of such processes include processes ofpackaging food products in which batch and date information is appliedto the packaging. In processes in which the packaging is formed from apolymeric film (e.g. sweet wrappers) the film passes rapidly from theprinting station to a position at which the product is wrapped. In suchprocesses it is important that the ink composition is substantially drybefore the wrapping process begins, since if it is not then smudging oreven removal of the printed indicia can occur. It is also important thatthe ink has a composition which adheres well to the substrate, since ifit does not, in this case also smudging and removal can occur.

Particular problems arise with substrates which are based upon polymersof monomers which include propylene (polypropylenes) or polymers ofmonomers which include ethylene (polyethylenes). Adherence to thesesubstrates is difficult and is made more difficult when, as is common,they are treated with a release agent (often necessary for subsequentprocessing of the film to form a product wrapping).

Therefore it would be desirable to be able to provide an ink compositionwhich is capable of rapid drying and is also capable of being formulatedto include binder materials which give good adhesion to substrates, inparticular the polypropylene- and polyethylene-based substratesmentioned above.

It has been found in the past that solvent components such as methylethyl ketone (MEK) and acetone have high drying rate and are thussuitable for use as solvents in compositions which are required to dryrapidly. However, it has been well known for many years thatcompositions which are formulated to dry rapidly on application to thesubstrate can give other problems. In particular, rapidly drying inkcompositions can lead to problems with blocking of the nozzles, and inkdrying in the gutters, of the printing equipment used to apply the inkto the substrate. For this reason inks which include fast dryingsolvents such as MEK have routinely included humectants and additionalsolvents known to have higher boiling point, in order to prevent this,which in turn slows down the drying time.

It would be desirable to be able to solve the above mentioned drying andadhesion problems without introducing additional problems of blockingand crusting in the printing equipment. WO92/14794 is concerned with theproblem of providing a rapidly drying ink which dissolves commonly-useddyestuffs and is particularly concerned with the problem of replacingthe commonly-used solvent MEK. It is also concerned with providing anink having suitable viscosity. WO92/14794 uses a carrier mediumcomprising a lower alkyl ester of a lower alkyl carboxylic acid andgenerally also a lower alkanol. Alkyl moieties for these materials oflength C1 to C5 are mentioned generally but the majority of the examplesinclude ethyl acetate, which is the highly preferred alkyl ester for usein WO92/14794. Ethyl butyrate and butyl acetate are also used in otherexamples.

U.S. Pat. No. 5,990,197 discloses ink formulations generally containinga near infrared fluorophore copolymerised with a polyester, a binder, anorganic solvent soluble electrolyte and an organic solvent selected froma C₃–C₆ aliphatic ketone, a C₃–C₆ aliphatic ester, a lower aliphaticalcohol or combinations thereof. The example inks use 2-butanone orethanol or a mixture of the two as the solvent. Methyl acetate ismentioned in a list of C₃–C₆ aliphatic esters but of these ethyl acetateis said to be preferred.

GB-A-2030075 discloses an ink jet printing process using opaque inks.The ink contains resin or polymer dissolved in a solvent. A wide rangeof solvents is disclosed, and a list is given reciting aliphaticalcohol, ketones, aldehydes, ethers, esters, glycols, glycol ethers,hydrocarbons and lactones. Methyl acetate is mentioned in a list ofester-type solvents but in the examples the solvents used are methanolor mixtures of methanol with MEK or mixtures of methanol, MEK andacetone.

SUMMARY OF THE INVENTION

According to a first aspect of the invention we provide an inkcomposition suitable for use in an ink jet printer comprising colourant,binder, and solvent which comprises methyl acetate. We also provide, ina second aspect, a method of printing onto a substrate comprisingapplying an ink composition to the substrate by an ink jet printingmethod, in which the ink has a composition which comprises colourant,binder, and solvent which comprises methyl acetate.

Surprisingly, we find that the use of methyl acetate as a component ofthe solvent allows the production of a composition which simultaneouslysolves the problems mentioned above. The ink dries sufficiently rapidlyto prevent smudging and removal of indicia but does not lead to problemsof blocking and crusting of printing equipment and also dissolves thematerials preferred for obtaining good adhesion to substrates such aspolypropylenes and polyethylenes.

Furthermore, we have found that the composition of the invention allowsfor the use of a wide range of resins and binder polymers. In particularit permits the use of a wider range than would be possible with acetoneand MEK.

In addition, we find surprisingly that compositions according to theinvention give faster drying time than compositions in which the knownfast-drying solvent MEK is used as the only solvent and even faster thansome MEK/acetone blends.

In the invention the solvent comprises, as an essential component,methyl acetate. This solvent component has the advantages, discussedabove, of fast drying time without inducing nozzle blocking and guttercrusting problems. It is also capable of dissolving a wide range ofbinder polymers, especially acetal polymers such as polyvinyl butyrals(PVBs), which are particularly beneficial for provision of adherenceproperties. We have found that other fast-drying solvents such asacetone and MEK do not dissolve PVB binders except under stringent andharsh conditions unsuitable for use in printing processes.

Methyl acetate has the further advantage of a low photochemical ozonecreation potential (POCP) of 2.5. POCP is an arbitrary scale ofatmospheric chemical activity based on ethylene at 100 and very stableorganic materials at zero.

On this basis, we also provide further aspects of the invention asfollows.

In a third aspect of the invention we provide an ink compositionsuitable for use in an ink jet printer comprising colourant, binder andsolvent which comprises methyl acetate and from about 4% to about 22%,by weight based on total weight of composition, ethanol.

In a fourth aspect of the invention we provide an ink compositionsuitable for use in an ink jet printer comprising colourant, binder andsolvent which comprises methyl acetate and acetone.

In a fifth aspect of the invention we provide an ink compositionsuitable for use in an ink jet printer comprising colourant, binderwhich comprises an acetal polymer and solvent which comprises methylacetate.

The invention also provides methods of printing onto a substratecomprising applying an ink composition to the substrate by an ink jetprinting method using any of the ink compositions of the third to fifthaspects of the invention.

In a further aspect of the invention we provide a method of printingonto a substrate based on polymers selected from the group consisting ofpolymers of propylene, polymers of ethylene and polymers of propyleneand ethylene, comprising applying an ink composition to the substrate byan ink jet printing method, and the ink has a composition whichcomprises colourant, binder and solvent which comprises methyl acetate.

In a further aspect of the invention we provide a process for making anink composition of the invention comprising mixing at least part of thebinder and at least part of the solvent to form a first pre-mix,separately mixing all remaining components except colourant to form asecond pre-mix, combining the first and second pre-mixes and adding tothis mixture the colourant.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The methyl acetate is generally present in the composition in an amountof at least 1%, preferably at least 2%. Especially preferred amounts areat least 5%, more preferably at least 8%. The methyl acetate can bepresent in any amount, and preferred levels are up to 70%, but generallyis not present in amounts of more than 60%. If methyl acetate is used inrelatively low amounts, the preferred upper limit is 20%. However, itcan be used, in some formulations, in higher amounts of for instancefrom 30 to 70%, preferably 40 to 60%.

In this specification, unless otherwise stated, percentages are byweight based on total composition.

Preferably the solvent is a blend and also comprises acetone. Whenpresent, acetone is preferably used in amounts of at least 2%, and thepreferred upper limit is 70%. If used in relatively low amounts acetonecan be present in amounts of from 2 to 20%, preferably 6 to 18%.Preferably however acetone is present in relatively high amounts of from30 to 70%, preferably 40 to 60%.

The composition can comprise ethanol as a cosolvent, generally added inthe form of denatured ethanol such as industrial methylated spirit (IMS)which comprises ethanol and low levels (around 4%) methanol, or DEB100,which contains Bitrex denaturant. When present, ethanol is preferablyused in an amount of from 4 to 22%, preferably 8 to 17%.

A particularly preferred solvent blend comprises methyl acetate, acetoneand ethanol (generally in the form of IMS).

When the preferred solvent blend of methyl acetate/acetone/ethanol isused preferred amounts, based on total solvent blend, are methyl acetate5 to 25%, preferably 10 to 20%/acetone 50 to 90%, preferably 60 to80%/ethanol 5 to 25%, preferably 10 to 20%.

The composition may contain other known solvent materials such as otheralkyl acetates, other alkyl esters of alkanols, other alkanols and MEK.Preferably however the composition contains substantially no alkylacetates other than methyl acetate. It is also preferred that thecomposition contains substantially no MEK, in particular when the bindercomprises an acetal polymer such as polyvinyl butyral.

An advantage of the formulation of the invention is that it gives fastdrying but without nozzle blocking and gutter crusting problems withoutthe necessity of including humectants or high boiling solvents, whichhave in the past been routinely used in combination with knownfast-drying solvents such as MEK in order to prevent these problems.Thus according to the invention the composition preferably containssubstantially no humectant or high boiling solvent. Humectants and highboiling solvents include high boiling glycol ethers such as propyleneglycol methyl ethers, N-methyl pyrrolidone, triacetin, amyl acetate(boiling point 142° C.) and methyl cellosolve (boiling point 125° C.)and others may be chosen as desired.

The total solvent or solvent blend generally makes up at least 60%,preferably at least 70%, often at least 85%, of the total composition.

The composition of the invention additionally comprises a binder.Binders may be selected from any of those known in the field. They aregenerally polymeric and include modified acrylic polymers, cellulosicmaterials, acetyl polymers, ketone resins, vinyl resins, acrylatepolymers, acrylic copolymers, aldehyde resins, phenolic resins, phenolicmodified resins, maleic resins, maleic modified resins, polyesters,rosins, rosin esters, modified rosins and modified rosin esters. Therosins and rosin esters (modified or unmodified) are preferably used inan amount of from 2 to 10%.

The binder may be a blend or may be formed from a single bindermaterial. Preferably the binder comprises an acetal polymer, forinstance polyvinyl butyral (PVB), alone or with additional bindermaterials.

When the binder comprises polyinyl butyral it is preferably a gradewhich contains a relatively high percentage of butyraldehyde. Inparticular, the PVB preferably has a butyraldehyde content of at least70%, more preferably at least 75%, in particular at least 80%. It can bepreferred that the butyraldehyde content is at least 85%. Percentagebutyraldehyde is specified by the manufacturer.

When the butyraldehyde content is relatively high, for instance morethan 80% and in particular at least 85%, it can be preferred that themolecular weight of the PVB is relatively low. For instance it may havea solution viscosity below 1000, in particular below 400 and in somecases below 300 cp when determined on a 15% by weight solution in 60:40toluene:ethanol at 25° C., using a Brookfield viscometer. Alternativelysolution viscosity may be not more than 300, preferably not more than200 and in some cases not more than 100 cp, when determined on 10%solution in 95% ethanol at 25° C. using an Ostwald-Cannon-Fenskeviscometer.

Examples of commercially available PVB resins are Pioloform BN18available from Wacker, Mowital B30HH and Mowital B60HH available fromHoechst AG and Butvar B79 available from Solutia.

Total binder level is preferably at least 2%, often at least 4%.Generally it is not more than 30%, preferably not more than 25%. Apreferred range is 5 to 20%.

The composition also includes a colourant. The colourant may be a dyesoluble in the composition or a pigment, optionally modified to renderit suitable for inclusion in the composition. A preferred class ofcolourant is chrome based azo dyes, but any suitable colourant may bechosen.

Colourant is generally present in amounts of at least 1%, often not morethan 15%, and the amount is preferably in the range 3 to 10%.

The composition may contain any other known components of inks suitablefor ink jet printing. These include adhesion promoters, for instanceTilcom (trade name) and alkoxysilanes, preferably in an amount of from 2to 10%; wetting agents and surfactants, for instance silanes andsiloxanes, preferably in an amount of from 0.2 to 15%; plasticisers, forinstance phthalates, preferably in an amount of from 0.8 to 3%;conductivity agents, such as quaternary ammonium salts, lithium nitrateand thiocyanates, preferably in an amount of from 0.2 to 4%; otheradditives for controlling stability and pH, for instance acrylichomopolymers and copolymers, preferably in an amount of from 2 to 8%.

Preferably all components are compatible with food products, since apreferred application of the composition is to wrappers for foodproducts. The composition and its components may be food grade.

The ink compositions may be made up in any known manner. However, apreferred method is as follows and is a further aspect of the invention.In the process of forming the ink composition at least part of thebinder and at least part of the solvent are mixed to form a firstpre-mix, all remaining ingredients except colourant are mixed to form asecond pre-mix and the first pre-mix and second pre-mix are thencombined. The colourant is subsequently added.

This process is particularly valuable when the solvent comprises ethanolsolvent and/or the binder comprises an acetal polymer such as PVB.

In this process it is preferred that some or all of the methyl acetateis included in the first pre-mix. For instance up to 90%, or up to 70%or 60% of the methyl acetate may be included in the first pre-mix. Ifacetone is used as part of the solvent then preferably at least someacetone is included in the first pre-mix, for instance up to 60%, 70% oreven 90% of the total amount of acetone in the composition. When ethanolis included in the composition it is preferred that all of the ethanolis included in the first pre-mix.

The first pre-mix is preferably made as follows. A proportion of thebinder, for instance up to 50% or even up to 70% or 80%, of the binderis mixed with all of the ethanol. Methyl acetate and acetone if used arethen added. The remainder of the binder is then added and the componentsmixed.

In the invention we also provide a method of printing on to a substrateby an ink jet printing method using the ink of the invention. In themethod, the composition may have any of the preferred features discussedabove in connection with the inks of the invention.

In the method, the composition is applied to a substrate. The inventionis particularly beneficial in application to substrates which havehitherto been regarded as difficult to print, for instance due toadhesion problems. In particular, the system of the invention, includingmethyl acetate as solvent, allows the use of acetal polymers such as PVBas the binder. Thus the invention is especially beneficial when thesubstrate is polymeric, for instance a polymeric film. Preferably thefilm is based on a polypropylene or a polyethylene.

The invention gives particular advantages when the substrate is apolymeric film which has been treated with a release agent. This releaseagent is often necessary for subsequent processing of the film butincreases problems with adhesion. In the invention it is possible tocombine fast drying and adhesion to substrates of this type.

In the process the ink composition generally dries rapidly afterapplication to the substrate, preferably in not more than one second,more preferably not more than 0.6 seconds.

Thus the invention is beneficial in systems where the substrate issubjected to further processing steps, such as formation into a productwrapping, within one second, preferably within 0.6 seconds, afterprinting.

The composition may be applied by any known ink jet printing method. Insuch methods an ink jet printer projects a stream of ink droplets from aprinter head to impact on the substrate as the substrate is conveyed,usually at high speed, past the printer head. The droplets arecontrolled so that they are deposited in a controlled array and thesubstrate is thereby printed with the desired indicia. The indicia mayfor instance be code numbers and letters, dates such as “sell by” datesand other alphanumeric data.

The method may be the known “drop-on-demand” method in which inkdroplets are generated as required to produce the predetermined array.

Preferably however the method is the known continuous ink jet printingmethod. A continuous stream of droplets is supplied and the droplets arediverted electrically in order to produce the predetermined array.Droplets which are not deposited on the substrate are collectedautomatically and recycled to a return tank.

Such methods are well known.

The invention will now be illustrated with reference to the followingexample.

EXAMPLE

The following is an example of a composition according to the invention.It may be applied to a substrate such as polyethylene or polypropylenefilm by a continuous ink jet printing method.

Acetone 53.8 IMS 13.0 Methyl acetate 12.2 Pioloform BN18 ¹ 7.0 Surcol860W2 4.0 Cellolyn 21E3 4.0 Butyl benzyl phthalate 1.5 Silwet L76224 0.5Orasol RLI5 4.0

-   1. Polyvinyl butyl binder supplied by Wacker.-   2. Acrylic copolymer supplied by Ciba Specialty Chemicals.-   3. Synthetic rosin ester supplied by Hercules.-   4. Polyalkyleneoxide modified polydimethylsiloxane supplied by    Witco.-   5. Black 6 isomer chrome based azo dye.

1. An ink composition suitable for use in an ink jet printer comprising(a) colourant, (b) binder and (c) solvent which comprises methylacetate, acetone, and ethanol, wherein the amount of methyl acetate is2–20% by weight based on total weight of composition.
 2. A compositionaccording to claim 1 which contains acetone in an amount of from 2 to70%, based on total weight of composition.
 3. A composition according toclaim 1 containing ethanol in an amount of from 4 to 22%, based on totalweight of composition.
 4. A composition according to claim 1 in whichthe binder comprises one or mare polymers selected from the groupconsisting of modified acrylic homopolymers or copolymers, cellulosicpolymers, aceral polymers, ketone resins, vinyl resins, acrylatepolymers, acrylic copolymers, aldehyde resins, phenolic modified resins,maleic modified resins, polyesters, rosins, modified rosins, rosinesters and modified rosin esters.
 5. A composition according to claim 4in which the binder comprises an acetal polymer.
 6. A compositionaccording to claim 5 which comprises polyvinyl butyral in an amount offrom 5 to 20%, based on total weight of composition.
 7. A compositionaccording to claim 1 which contains substantially no acetate estersother than methyl acetate.
 8. A composition according to claim 1 whichcontains substantially no methyl ethyl ketone.
 9. A method of printingonto a substrate comprising applying an ink composition to the substrateby an ink jet printing method, in which the ink has a composition whichcomprises solvent, which comprises methyl acetate, acetone, and ethanol,wherein the amount of methyl acetate is 2–20% by weight based on totalweight of composition.
 10. A method according to claim 9 in which thesubstrate has been treated with a release aid.
 11. A method according toclaim 9 in which the ink is substantially dry in not more than 0.6seconds, after application to the substrate.
 12. An ink compositionsuitable for use in an ink jet printer comprising (a) colourant, (b)binder and (c) solvent which comprises 2–20% methyl acetate, by weightbased on total weight of composition, 4% to 22% ethanol, by weight basedon total weight of composition, and acetone.
 13. An ink compositionsuitable for use in an ink jet printer comprising (a) colourant, (b)binder which comprises an acetal polymer and (c) solvent which comprisesmethyl acetate, acetone, and ethanol, wherein the amount of methylacetate is 2–20% by weight based on total weight of composition.
 14. Aprocess of making an ink composition which is suitable for use in an inkjet printer and which comprises colourant, binder and solvent in whichthe solvent comprises methyl acetate, acetone, and ethanol, wherein theamount of methyl acetate is 2–20% by weight based on total weight ofcomposition, the process comprising mixing at least part of the binderand at least part of the solvent to form a first pre-mix, mixing theremaining components except the colourant to form a second pre-mix,combining the first pre-mix and the second pre-mix to form a mixture andadding the colourant to the mixture.
 15. A composition according toclaim 1 which contains acetone in an amount of from 30 to 70%, based ontotal weight of composition.
 16. A composition according to claim 1containing ethanol in an amount of from 8 to 17%, based on total weightof composition.
 17. A composition according to claim 1 which comprisespolyvinyl butyral.
 18. A method according to claim 9 in which the ink issubstantially dry in not more than one second.